The present invention relates to an improved valve structure and, more particularly, to a shuttle valve for enabling the selective supply of fluid from alternate sources through a single outlet.
In the prior art, shuttle valves are generally used to control the application of fluid to fluid-actuated mechanisms when it is desired to provide fluid from one of two alternate sources at any given time. Such a control arrangement usually employs a shuttle valve structure which provides fluid from a primary source to an outlet port until the pressure of the primary source decreases to a particular value. At that time, a secondary source is coupled to the outlet port by valve action. A shuttle device of this type, however, encounters several problems when equal pressures are supplied from both fluid sources. Thus, when the system must initially be operated with fluid sources at the same or nearly equal pressures, the shuttle may oscillate causing intermittent fluid flow from each source or stabilize at an intermediate position effectively shutting off the flow of fluid from both sources. While various techniques have been proposed to overcome such problems, none has successfully achieved stable operation without significant disadvantages.
By way of example, one such shuttle valve incorporates a detent configuration which enables the shuttle to be seated in only one of two positions in order that flow from only one of the sources may be provided to the outlet at any given time. In addition to being more complex, such a configuration requires the expenditure of a threshold level of pressure to overcome the force of the particular detent involved. While the configuration may prevent the shuttle from stabilizing at an intermediate position, the same also restricts use of the shuttle device to systems having the particular pressure characteristics capable of providing detent shuttle action.
In regard to the problem of oscillation, the prior art has included a spring bias shuttle configuration wherein a spring causes the shuttle to normally remain seated in one position and thereby provide fluid flow from a primary supply to an outlet port. Following a decrease in fluid pressure at the primary source, the secondary source, having a pressure sufficient to overcome the spring bias, causes the shuttle to shift to a second position such that fluid from the secondary source will be coupled to the outlet port. With this technique, however, one of the fluid sources must expend a significant amount of energy to keep the shuttle positioned against the spring bias in order to enable fluid flow from that source to the outlet. In those instances where such a source is of limited energy, the energy loss caused by fluid biasing of the spring loaded shuttle may be detrimental to system operation. Thus, the use of the shuttle is again restricted by its particular configuration to systems of fluid supply having specific pressure or energy characteristics.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide a versatile shuttle valve for enabling fluid control under a variety of conditions.